Radio frequency (RF) transmitters (TX) and receivers (RX) use RF local oscillators (LO) driving mixers that in turn are used to up-convert and down-convert signals. The LO signals, generated by frequency synthesizers such as phase-locked-loops (PLLs), exhibit phase noise and that ultimately limits the attainable performance of a radio link. The problem with phase noise is two-fold. Firstly, the phase noise at smaller frequency offsets from the LO frequency generally leads to a limit on error-vector magnitude (EVM) and in the case of orthogonal-frequency division multiplexing, OFDM, transmission it is readily understood that sub-carriers will interfere with one another. Secondly, phase noise at higher offsets is generally a problem in radio communication systems, like cellular radio, with very crowded spectrum and where a strong signal, adjacent in frequency to a desired signal, will mix with the phase noise and generate co-channel interference, which is commonly referred to as reciprocal mixing. Thus, the issue of phase noise needs to be considered.
Designing hardware for radio communication using mm wavelength frequencies (e.g. 60 GHz), especially battery operated user equipment or low power access nodes, is challenging for many reasons. LO phase noise is one such aspect that when compared with cellular radio standards or WiFi, e.g. 802.11n,ac, etc., operating in the 0.5-5 GHz regime it will be much worse in the mm wavelength regime. For a given power consumption budget for a controlled oscillator the phase noise in dBc/Hz at a given offset roughly scales with 20*log(fLO). To obtain decent phase noise performance from an LO synthesizer, incorporating a controlled oscillator, CO, providing an oscillating signal at a frequency based on a control signal, without increasing CO power consumption excessively compared with the low-GHz regime, one can increase the loop bandwidth of the LO synthesizer. Increasing the LO synthesizer loop bandwidth will also lead to faster frequency lock and tracking behaviour. On the other hand, this will increase the requirements on the frequency reference generator, typically a crystal oscillator accordingly such that its associated power consumption may reach that of the LO synthesizer or even become larger, on the order of 100's of mW instead of a few mW as is common for the low-GHz range user equipment. Such high reference oscillator power consumption is not desired in battery operated equipment or in other equipment on a low power budget.
Circuits designed for very low phase noise may have relatively high power consumption compared with circuits where more phase noise is accepted. It is therefore a desire to provide an approach providing both acceptable phase noise and relatively low power consumption.